Sandro Galea: One of the biggest factors was people who were actually exposed to the event. So people who actually saw the towers fall, people who lost possessions in the event, people who had family members who died, that was a big risk factor for having post traumatic stress symptoms. As I said, we talked to the general population, but of course in the general population you have a number of people who've had direct experience with the event. In fact, we found that about a quarter of people in the general population we could call directly affected, who had any of a number of different exposures. So being directly affected was a big risk factor.

Being Latino was a risk factor. So Latinos in New York City had about a two-fold greater risk of having post traumatic stressthan did either whites or African Americans or Asians. We found that people who had symptoms of panic around the time of the event - so that means people who had heart palpitations, they were breathing quickly, felt like they were going to die at the time of the event - were more likely to have post-traumatic stress subsequently. And also people who had previous mental health problems, people who had prior stressors before the event. And that probably represents a group of people who were vulnerable to having psychological symptoms when a traumatic event happens.

Sasha Aslanian: It sounded to me like you had some really interesting findings about how the rates differed in the different boroughs. I'd love to hear about that and also, what you think might account for that.

Galea: Yeah, when we started our work, we assumed that rates of post-traumatic stress might be higher in Manhattan than anywhere else because it was an event in Manhattan, after all. But we subsequently looked at the rates by boroughs and found that that was actually a wrong assumption. The prevalence of post traumatic stress was actually higher in Brooklyn and in the Bronx.

There were different reasons that accounted for that. In Brooklyn, it was primarily because a lot more people were actually directly affected by the events in Brooklyn because you could actually see the event much more clearly in Brooklyn and a lot of commuters were from Brooklyn. In the Bronx, it was primarily driven by the Hispanic issue that we talked about, you know the Bronx is heavily Hispanic and the likelihood of post-traumatic stress among Hispanics was substantially higher than among other ethnic groups, so the Bronx as a whole has a higher rate than Manhattan.

So that was a little bit of a surprise for us. We didn't expect that and I think what it tells you, is that in disasters like this, which are happening in a very dense area, there are multiple factors that come into play, and these factors have to be weighed against each other and distance and exposure to the event is weighed by ethnicity and job loss, previous traumas, subsequent traumas, making for a pretty complicated pattern of who ends up having symptoms and what the trajectory is of symptoms over time.

And the other thing we found is that the factors that predicted whether you kept your symptoms, the factors that predicted the trajectory of symptoms, were different than the factors that predicted who got symptoms to begin with. So for example, we said that mental health problems before September 11 were important predictors of getting post-traumatic stress. Well now those mental health problems were not a factor of keeping it.

What was a predictor of keeping post-traumatic stress was new stressors. So that means that people in the first six months that September 11 happened, and then life goes on, and then other things happen in life - people's parents die, people are involved in traumatic car accidents - it is those people who now were at risk of keeping the post-traumatic stress.

And the reason that's important is because it means that if we're looking to target treatment, if we're looking to identify risk groups, the risk group is changing. Six months seems like a long time, but in fact, in setting up treatment, it takes time. So the problem is that if we're chasing the risk factors that we know are a problem early on, and designing treatments around those risk factors, by the time we've sort of set up treatment systems and screening systems and ways to identify people, the risk factors for who we should be chasing have actually changed. So that has substantial implications I think for the public mental health system and I'm not sure that we've fully grappled with those yet.